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Confession: I’ve been caught by a red-light camera. It’s pretty much the only smudge on an otherwise spotless driving record (although I’m not sure if that’s a testament to my skills behind the wheel or just sheer luck). Either way, receiving a ticket in the mail from the city of Chicago with a close-up shot of my frazzled, late-to-an-interview face really stunk. Especially when it cost me $100 and I didn’t even get the job.

While the effectiveness of red-light cameras is often up for debate, red-light running is one of the top causes of accidents, injuring over 165,000 people per year, killing hundreds, and costing about $7 billion in property damage, medical bills, and insurance increases.

A 2011 study by the Insurance Institute for Highway Safety found that large cities with red-light cameras saw a 24 percent decrease in fatalities caused by red-light runners. The fact that running red lights still accounts for 22 percent of traffic accidents makes the demise of red-light cameras seem pretty unlikely.

So, if red-light cameras are here to stay, let’s take a look at how they actually work. What cues the cameras to snap that oh-so-flattering headshot of the driver, followed by a close-up of their license plate? Are they just endlessly snapping, hoping to catch rule-breakers in the act? Believe it or not, there’s actually a lot of science behind those eyes in the sky.

The triggers

Although we’ve all noticed the ominous cameras keeping watch over our intersections, the process actually starts underground. While trigger technology varies, most red-light systems use induction loops buried in the asphalt. Induction loops are made by layering 2 electrical wires on top of each other in rectangular loops. The wires are connected to a power source that supplies electricity and a meter that measures the current running through the loops.

If you had the pleasure of sitting through physics class in your formative years, this should all sound pretty familiar. But in case you “missed that day” (or opted to take botany, ahem), the power source creates a magnetic field, sending a constant, measureable flow of electricity through the wires.

When the light turns red, the system turns on. The meter monitors the current in the wires, and if a massive hunk of metal rolls over those wires, the meter will detect the shift in electric flow. In order for the meter to send a signal to the computer operating the camera, the car must be traveling at a particular speed to avoid ticketing drivers who are simply stopped on top of the loop.

Caught on camera

Typically, red-light systems have cameras at all 4 corners of an intersection, ready to catch traffic violators at different angles. If the trigger sends a signal to the computer that a car has passed through the induction loop, the camera automatically starts snapping.

The first photo will show the car entering the intersection, while the second shot catches the driver passing through the crossroads, focusing on their license plate. The computer then records the date, time, intersection, speed, and elapsed time from when the light first turned red and attaches it to the image.

With the specifics of your slipup detailed on the photo, the police have all the evidence they need to look up your address, drop your photo and fine in an envelope, and send the little surprise your way.

“But I wasn’t driving!”

So what happens if you receive a red-light camera ticket, but it’s a case of mistaken identity (say, your good friend Adam was breaking the rules in your ride)? Like any other ticket, you have the right to contest the charges in court. Unfortunately, in some states, red-light cameras only capture the license plate. While you’re still able to contest the ticket, you’ll likely have a harder time proving it wasn’t you.

If you can prove that the person behind the wheel wasn’t you, a judge can decide to throw the ticket out, effectively saving you the fine, any points issued, and a possible jump in your insurance rate. (And if you’re an Esurance policyholder, you can check out our handy What If® Calculator to see how a ticket or accident-related claim could impact your rates.)

The aftermath

Since paying my debt to the city of Chicago, I’ve been much more cognizant of the amount of time I need to make it through an intersection. The reality is I got off easy. Causing an accident and injuring myself or others would have been infinitely worse. Being punctual is great, but it’s better to be a few minutes late and arrive in one piece.

Share your story

Have you ever been caught running a red light on camera? Did the experience teach you a lesson in taking your time? Tell us about it in the comments below.

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about Jennifer

A Motor City native, Jennie spent a few years knee-deep in automotive before heading to San Francisco to join the Esurance team as a copywriter. In addition to words, she’s a big fan of running, Detroit Tiger baseball, and the Trader Joe’s cheese aisle.